How do I merge multiple animated Armatures?
Do the following:

Copy all armatures in object mode and merge by ctrl+j. You will lose animation data, but this not a problem, we will copy keyframes from separate ones.

Enter the Pose Mode and add one keyframe to all bones. We need this to get animation tracks.

Now move to the dope sheet editor and copy all keyframes from the separate armatures. When select new one and paste keyframes there.

Now you don't need old armatures, remove them